This invention relates to an improved process for making block copolymers of vinyl aromatic hydrocarbons and/or conjugated dienes and polydimethylsiloxane. More specifically the invention relates to the use of specific crossover reagents to speed up the process for making such block copolymers.
Linear block copolymers of polystyrene and polydimethylsiloxane have been synthesized, both by graft and block copolymerization. In block copolymerization of such linear polymers, polystyrene is produced by anionic polymerization with an organo lithium promoter and the living polymer (PS.sup.- Li.sup.+) created thereby is reacted with hexamethylcyclotrisiloxane, (Me.sub.2 SiO).sub.3, in the presence of a polar ether promoter wherein a block of polydimethyl-siloxane grows on the end of the living vinyl aromatic hydrocarbon polymer block.
The kinetics of the polymerization reaction of styrene are quite good and the reaction proceeds relatively quickly. Unfortunately, the crossover reaction for the living polystyrene polymer blocks to begin the growth of the polydimethylsiloxane blocks on the ends thereof exhibits considerably less favorable reaction kinetics, causing the reaction to be quite slow. This is the reaction from PS.sup.- LI.sup.+ to PS--[Si(Me).sub.2 O].sub.2 --Si(Me).sub.2 O.sup.- Li.sup.+. This reaction generally requires a large excess of hexamethylcyclotrisiloxane monomer--a monomer to lithium ratio on the order of 100 to 1--to drive it to completion. The large excess results in significant die-out of the living polystyrene blocks due to impurities in the siloxane monomers, thereby generating homopolystyrene and possibly adversely affecting the desired molecular weight for the polydimethylsiloxane block copolymer.
Thus, it would be advantageous to provide an improved process which does not require a large excess of hexamethylcyclotrisiloxane monomer to decrease the cost and to minimize the die-out of the polystyrene blocks. The present invention provides such an improved process.